Using a method called two-dimensional infrared spectroscopy, they measured and compared the wavelength of light emitted by white blood cells from 50 people known to have different stages of Alzheimer's, and 20 healthy volunteers.

They found a strong link between the type of light emitted by the cells and the severity of Alzheimer's. This was due to the different stages of formation of the amyloid-beta remnants, called peptides.

Writing in the journal Analytical & Bioanalytical Chemistry, the authors, led by Pedro Carmona of the Instituto de Estructura de la Materia in Madrid, concluded: "Our measurements seem to be more sensitive for earlier stages of Alzheimer's disease, namely mild and moderate."

This is not the first potential blood test for Alzheimer's to be investigated.

British researchers at King's College London are working on one based on levels of a protein called clusterin, while scientists in the US and Finland are looking at other methods.

Dr Laura Phipps of Alzheimer's Research UK, said the Spanish technique "could hold information useful" for detection, but cautioned: "This work is still in its very early stages. It takes many years to develop a diagnostic test that can be used in the clinic, and we would need much more research to see whether this method has potential."

Dr Anne Corbett, from the Alzheimer's Society, added: "We do not know whether the changes to the proteins are a cause or a symptom of the disease or whether they can be detected before symptoms develop.

"One in three people over 65 will die with dementia, yet dementia research is still drastically under-funded. We must invest now."